1. Field of the Invention
The present invention relates generally to a hydraulic control system for an automatic automotive type transmission and more specifically to a lock-up arrangement for use with such a system.
2. Description of the Prior Art
JP-A-57-33253 discloses a lock-up control arrangement which includes a lock-up apply chamber and a lock-up release chamber. A valve spool which is associated with the two chambers has first and second pressure responsive areas which are respectively exposed to the pressures in the apply and release chambers.
This arrangement however has suffered from the drawbacks that as the first and second pressure responsive areas of the spool have different areas, in the event that the pressure in the apply chamber suddenly increases the pressure differential between the apply and release pressures (which produces the clutch application force) cannot be maintained at a desired response target level. As a result the driving and ride characteristics of the vehicle tend to be deteriorated.
That is to say, during slip lock-up, the pressure differential is controlled by a lock-up solenoid pressure P.sub.SOL.L/U and the equilibrium is established in the manner shown in FIG. 8 wherein it can be shown that: ##EQU1##
Accordingly, during slip lock-up it can be shown that .DELTA.P is given by: ##EQU2##
It should be noted that from equation (1) that the lock-up solenoid pressure P.sub.SOL.LU is controlled in a manner to appropriately adjust the pressure differential .DELTA.P and that .DELTA.P varies as a function of the solenoid pressure and the apply pressure. Viz., .DELTA.P=f(P.sub.SOL.LU,P.sub.T/A).
Accordingly, when the amount of depression of the accelerator pedal changes during slip lock-up, the level of the apply pressure P.sub.T/A changes (with respect to line pressure P.sub.L) thus changing the value of .DELTA.P. As a result, as the level of the solenoid pressure singularly determines .DELTA.P it has been noted by the inventor that the following detrimental effects are encountered.
[1] If a shift takes place during slip lock-up, the level of the line pressure varies either as a result of the sudden changes in engine speed or a deliberate switching to a different line pressure control schedule, and thus renders it impossible to maintain the desired .DELTA.P level. As a result of this, lock-up is inappropriately induced. This produces vibration in the lock-up damper and deteriorates the shift and ride characteristics.
[2] during slip lock-up, if the accelerator pedal pumped, the feedback control of the engine and turbine rotational speeds cannot be accomplished and the amount of slip deviates from the desired target value.
[3] When the engine is operating in a low speed zone and the output of the A/T oil pump is accordingly low, slip lock-up is initiated, when the engine load is increased the engine speed tends to decrease with the result that the line pressure exhibits an attendant decrease. The .DELTA.P value also exhibits a reduction and the amount of slip resultingly increases. As a result of this slip increase, the engine speed also tends to increase and leads to an increase in the line pressure level. The increase in line pressure level also induces an increase in the .DELTA.P value which tends to return toward the desired level.
That is to say, during the initial stages of slip lock-up, the amount of slip increases and decreases (viz., hunts up and down) for a short period. This of course results in a deterioration of passenger comfort.
When the level of .DELTA.P is controlled by the level of the solenoid pressure P.sub.SOL.L/U the pressure responsive surface area on which the solenoid pressure acts is set a relatively large value whereby a small changed in the solenoid pressure results in large change in the .DELTA.P value. This leads to the following problems:
[4] The inevitable unit-to-unit deviation in solenoid valves and in the duty cycles of the signal which are applied thereto, induces a very important need to obviate the effects of these phenomenon.
If it is possible to permit a relatively large variation in the solenoid valves, it is possible to achieve a notable reduction in inspection and associated costs. However, if the control gain is set at a high value, the deviation in control tends to maximize it is vital that each solenoid valve exhibit a extremely small unit to unit variation.
[5] For example, if the lock-up solenoid pressure P.sub.SOL.LU changes by 0.1 kg/cm.sup.2 the value of .DELTA.P changes by the amount of: ##EQU3##
With this amount of change, when the vehicle is operating in a high speed zone at full open throttle, lock-up is induced, while during low load intermediate vehicle speed slip lock-up is induced. Viz., it is desired to assuredly maintain lock-up during full throttle modes of operation. At low throttle settings however, when slip lock-up is induced the amount of slip permitted is greatly increased.
Accordingly, even as a result of this very small 0.1 kgm/cm.sup.2 change in pressure the amount of slip changes ten fold.
Thus, when the slip mode is induced, the amount of slip cannot be expected to be feedback controlled a stable target value. Further, during the change from open converter operation to slip lock-up, as the control or adjustment range is extremely narrow, if the variation in the valve units is large, full lock-up is apt to occur and result in the above mentioned ride deterioration.